Fuel batteries have been expected to be put to practical use as electrical power generation sources having a high energy conversion rate and low environmental burden. A known example of the fuel batteries has a cell with a solid electrolyte membrane and a catalyst layer. There has been proposed a polymer electrolyte fuel battery employing a polymer electrolyte membrane as a solid electrolyte membrane and capable of operation at a temperature of as low as about 80° C. However, due to large volume use of expensive platinum (Pt) as a catalytic metal of the catalyst layer, cost has been a bar to practical use of the polymer electrolyte fuel battery.
For practical use of fuel batteries, reduction in usage of Pt has been proposed (e.g. in Patent Publication 1). However, the fuel battery free of Pt disclosed in Patent Publication 1 requires hydrazine (N2H4), which has pungent odor and is flammable, as the fuel, which imposes safety and other problems.
As a solution to this problem, there has been proposed a technology wherein a metal composite oxide is used as a solid electrolyte. However, since a solid electrolyte used in a fuel battery is generally required to have ion conductivity, use of a metal composite oxide requires a technology for expressing its ion conductivity.
In this regard, Patent Publication 2 proposes a fuel battery employing, as a solid electrolyte of a metal composite oxide, LaSr3Fe3O10 having Ruddlesden-Popper type structure and capable of achieving strong electromotive force even under the conditions of as low as about 20 to 80° C. The publication proposes to enhance the ion conductivity of the solid electrolyte used in this fuel battery by, for example, calcining a pre-baked LaSr3Fe3O10 pellet at about 1400 to 1500° C., and subjecting the calcined pellet to steam treatment, preferably hydrogen reduction and steam treatment, to intercalate water or hydroxyl groups.
Patent Publication 2 teaches that the steam treatment of the LaSr3Fe3O10 solid electrolyte is carried out preferably at 20 to 150° C. and 30 to 100% relative humidity under 0.1 to 1 MPa pressure for 3 to 48 hours, particularly preferably at 25° C. and 100% relative humidity under 0.1 MPa pressure for 3 hours.
On the other hand, Non-patent Publication 1 reports that thermogravimetric (TG) analysis may be employed for determination of the amount of water or hydroxyl groups intercalated into s solid electrolyte through the above-mentioned steam treatment. Specifically, the publication discloses that, in NdSr3Fe3O8.5 having water incorporated in its structure, the water is present as free water or hydroxyl groups, which correspond to the two steps of mass losses confirmed by TG analysis, i.e., mass loss beyond 90° C. representing departure of free water and mass loss at about 250° C. representing departure of hydroxyl groups.
The amount of water or hydroxyl groups intercalated into a solid electrolyte, which is determined by TG analysis, was indeed determined of the LaSr3Fe3O10 solid electrolyte disclosed in Patent Publication 2, and FIG. 15 therein shows that the mass loss over 20° C. to 400° C. was a little over 1%. Such a mass loss cannot represent a sufficient amount of water or hydroxyl groups intercalated into the solid electrolyte, and further enhancement of ion conductivity of a solid electrolyte is desired.    Patent Publication 1: JP-2006-244961-A    Patent Publication 2: WO 2010/007949    Non-patent Publication 1: D. Pelloquin et al. Chem. Mater. 1715-1724, 16 (2004)